Cardiac and respiratory motion are two of the most common sources causing motion artefacts during a MRI scan. For example, when a subject is breathing, the lungs and kidneys and other organs and tissues move involuntarily and create motion artefacts. These motion artefacts make it difficult to obtain data in the region of interest over time due to the movement of the tissue or organ in the region of interest. Taking the example of breathing, as a cause of breathing a kidney could move left or right in the coronal plane, up or down in the coronal plane or even in and out of the coronal plane. As a consequence, the nearby organs and vessels will be moving as well.
To reduce motion artefacts in MR images, information acquired concurrently in “navigator” signals can be used to correct the plane of acquisition. The navigator signals are produced by pulse sequences which are interleaved with the imaging pulse sequences and which are characterized by the absence of phase encoding. The navigator signal is thus a projection along an axis defined by the readout gradient which is fixed in direction throughout the scan, and therefore relates to one-dimensional motion. If the motion artefact is due to breathing motion, the navigator signal typically determines the respiratory phase by monitoring the movement of the diaphragm.
It is a disadvantage that motion correction is using such navigator signals does not take into account that the entire abdominal or thoracic region does not move similarly to the position of the navigator.
It is a further disadvantage that motion correction using such navigator signals does not take into account the elasticity of most tissue in the abdominal and thoracic region.